Draw a filled triangle in DrawingContext

This is a follow up from this question.

Question:

I am trying to draw a filled triangle using DrawingContext, which is rendered on a DrawingVisual

Currently, I have managed to draw the outline of a triangle using the following C# code:

private DrawingVisual CreateTriangle()
  {
     DrawingVisual triangle = new DrawingVisual();
     using ( DrawingContext dc = triangle.RenderOpen() )
     {
         Pen drawingPen = new Pen(Brushes.Black,3);
         dc.DrawLine(drawingPen, new Point(0, 50), new Point(50, 0));
         dc.DrawLine(drawingPen, new Point(50, 0), new Point(50, 100));
         dc.DrawLine(drawingPen, new Point(50, 100), new Point(0, 50));

     }
     return triangle;
  }

I get this:

How do I draw a triangle that, in addition to th border I have drawn also has a red fill?